Signal conditioning circuits are often utilized as an interface in a signal conditioning unit to convert a differential input signal received from a data source into a more usable output signal. Signal conditioning circuits can be utilized in conjunction with sensors or transducers to receive a sensor or transducer output signal and convert this signal into an output voltage or current or frequency or pulse width or pulse position in analog or digital form utilized by a control system. For example, in order to maintain the quality of air, it is desirable to control internal combustion engine emissions. It is therefore necessary to provide long cable connections between the sensors and the signal conditioning circuits, which as a result, suffers from a poor signal to noise ratio due to the “pick up” of electrical interference and noise in the connecting cable. The degree of such interference is a function of the length of the connecting cable, sensitivity of the sensor and the general character of the electrical load.
Generally in the sensors the signal conditioning is a front end analog electronics which is very critical and difficult to design as it has to have high sensitivity, good selectivity, and immunity to noise, low cost, reproducible and mass producible. Such signal conditioning circuits include a voltage amplifier or a charge amplifier or a current amplifier (electrometer). For example, exhaust sensors utilized for sensing particulate matter the measured charge can be in the order of less than or equal to a pico coulombs. The signal to noise ratio will be very poor and the most of the noise will be due to common mode noise pick up from power line and other sources.
In a majority of prior art signal-conditioning circuits the signal from the sensor can be processed with noise and in the signal conditioning unit the noise can be eliminated selectively. But this results in a saturation condition at signal condition stage if the noise level is sufficiently large. Also if the signal and noise frequencies are the same then selectively eliminating the noise becomes a great challenge, difficult and expensive. Similarly, the noise at the input of the signal-conditioning amplifier can also be eliminated which has to be implemented in the analog amplifier design. The charge amplifiers utilized in sensing less than or equal to a pico coulomb charges in the exhaust gas and/or in the engine cylinder can be embedded in the common mode noise. Common mode noise is a disturbance that affects a plurality of lines similarly (e.g., causing a change of voltage of similar polarity and amplitude on each of the lines).
Common mode noise may be induced upon differential lines by another line that is parasitically coupled to the differential lines. The lines may be conductors, for example conductors formed in a metal layer of an integrated circuit or wires. The lines can carry signals. The signals carried by the line that is parasitically coupled to the differential lines may affect the signals carried by the differential lines through the effect of the parasitic coupling. The biggest source of common-mode noise is the difference in potential between two physically remote grounds. The second most significant common-mode noise source is ungrounded sources. Common-mode rejection techniques can be implemented to prevent common-mode noise from being converted to normal-mode voltage.
FIG. 1 illustrates a block diagram representation 100 of a prior art co-axial cable 190 utilized for connecting sensor element 140 and differential amplifier (charge or voltage or current) 170. The signal from the sensor 140 can be connected to the differential amplifier 170 through the co-axial cable 190. The co-axial cable 190 includes an inner cable 120 and an outer shield 110. The differential amplifier 170 can receive an output signal 150 and 160 from the inner cable 120 and the outer shield 110. The differential amplifier 170 delivers an output signal 180, which is proportional to the difference between the voltages on its input 150 and 160. The signal induced due to external interference in the inner cable 120 is almost zero and the signal induced due to the external interference on the outer shield 110 is maximum because the outer shield 110 functions to shield the inner cable 120. As depicted in FIG. 1, a 50 Hz noise signal 130 can be picked up by the outer shield 110 and will be maximum, which results in an increase in noise level at the output 180 of the differential amplifier 170.
FIG. 2 illustrates a block diagram representation 200 of a prior art untwisted pair of cable 290 utilized for connecting the sensor element 140 and differential charge amplifier 170. Note that in FIGS. 1-3, identical or similar parts or elements are generally indicated by identical reference numerals. The differential auto zero offset charge amplifier 170 can receive an output signal 240 from a first wire 220 and another output signal 230 from a second wire 210 of the untwisted pair cable 290. The 50 Hz/mains power supply frequency or external noise signal 130 picked up by the untwisted/twisted pair cable 290 will be equal; hence an output 250 of the differential charge amplifier 170 to 50 Hz picked up noise signal 130 will be a minimum. A shielded, twisted pair can be utilized in a channel to connect a signal from a source to an input terminal. The shields minimize capacitive coupling and the twisted wires minimize inductive coupling.
FIG. 3 illustrates a block diagram representation 300 of a prior art untwisted/twisted pair of cable 390 with grounded shield 320 utilized for connecting sensor element 140 and differential amplifier 170. The shield 320 of the untwisted/twisted cable 390 can be grounded. The differential charge amplifier 170 can receive an output signal 340 and 350 from the untwisted/twisted pair of cable 390. The output 360 of the differential charge amplifier 170 is immune to 50 Hz/mains power supply frequency or external noise signal 130 will be minimum as charge induced in the cable 390 will be minimum and equal. The twisted or untwisted grounded shield cable 390 can be utilized to reduce the common mode noise signal.
Based on the foregoing it is believed that a need exists for an improved low noise differential charge amplifier utilizing twisted or untwisted two pair cable with grounded shield for measuring less than or equal to pico coulomb charge in noisy elevated temperature and corrosive environment.